Oscillations, Waves, and Interactions - GWDG

DPI60plus – a future with biophysics 445
Figure 8. Compression of single DNA tetrahedra. Compression curves show linear elastic
response up to a load of 0.1 nN. At higher forces, most tetrahedra deform irreversibly. Offsets
were adjusted to overlap the linear parts of the seven curves. Inset: Reversibility of the elastic
response of a typical tetrahedron. (Figure from Goodman et al., Science 2005, Ref. [8]).
We used these structurally braced tetrahedra to investigate the behaviour of DNA
under compression. Although DNA under tension has been widely studied [9–12],
DNA strands of micrometer length buckle at extremely low forces. To measure the
mechanical response of a single tetrahedron directly, the AFM tip was centered over
a tetrahedron first located in imaging mode and was then moved toward the surface
while recording force. Compression curves for seven distinct 3×20/3×30-bp tetrahedra
are shown in Fig. 8. For forces up to 100 pN, the response was approximately linear
and reversible (Fig. 8, inset) with an average force constant of 0.18(±0.07) Nm −1 .
At higher forces, the response was nonlinear and varied from tetrahedron to tetrahedron.
From the gradient of the linear part of the measured F-d curve, we infer an
elastic modulus of Kc = 0.7(±0.3) nN for one DNA double helix in compression [8].
3 Motor proteins studied by single-molecule fluorescence
and optical trapping
3.1 Introduction: Kinesin function and structure
Molecular proteins are enzymes which use ATP-hydrolysis to move cargoes along
cytoskeletal filaments (microtubules or actin-filaments). We study how members of
the kinesin family of motor proteins function in microscopic detail. The kinesin family
consists of several classes of motors which are structurally and kinetically diverse,
but share high similarity in the motor domain (or head), which is responsible for
ATP- and microtubule binding. In the case of conventional kinesin (Kinesin-1) two
identical kinesin heavy-chains (KHC) – bearing the head-domain on the N-terminal